The invention relates to a digital-based phase control system and more particularly to such a system capable of providing phase adjustment of an output signal with respect to a reference signal over a wide range of precisely controlled increments.
Phase control systems are known to provide an output signal which has a predetermined phase with respect to a reference signal. To obtain the foregoing it is generally known to utilize a phase-locked loop including a phase comparator and a voltage controlled oscillator (VCO). The output of the phase comparator is connected to the input of the VCO. The phase comparator has two inputs, one of which receives a reference signal and the other input receives an output signal from the VCO, which is generally referred to as the feedback signal. The phase comparator compares the phases of its two input signals and provides a phase error signal. In response to the phase error signal the VCO adjusts the phase of its output signal to minimize the phase error signal.
It is further known to control the phase of the output signal from the VCO to have a predetermined phase displacement with respect to the reference signal. In the known devices such phase displacement is obtained by utilizing analog or digital phase shifting or delay devices connected in the feedback signal path, or alternatively, in the reference signal path of the phase-locked loop. Known analog signal phase shifting or delay devices may include networks comprising resistors (R) and capacitors (C), or inductors (L) and capacitors (C), analog LC delay lines or analog shift registers implemented for example by charge-coupled devices. The desired phase displacement may be adjusted by varying the R, L or C parameters of the above networks or delay lines, by varying the length of the delay lines or by varying the frequency of a clock signal applied to the charge-coupled devices. However, the above-indicated analog phase shifting and delay devices are suceptible to drift due to aging and environmental changes, such as temperature and humidity. The drift must be compensated for by frequent operator adjustments and maintenance. While drift compensation circuits are known, they do not eliminate the problem entirely. In addition, most of the above-indicated devices are not suitable for remote control.
One type of known digital phase shifting or delay devices includes one or more monostable multivibrators including analog time-out circuitry comprising R and C or L elements, or a voltage-controlled constant current source replacing the R element. The time-out circuitry determines the duration of an output pulse whose trailing edge is used to determine a desired phase displacement. While it is possible to obtain phase adjustment by varying the R or C parameters of the time-out circuitry or by varying the control voltage of the current source, respectively, these devices are unstable.
Another type of digital devices includes one or more monostable multivibrators with digital time-out circuitry, for example utilizing counters. Phase adjustment of the output signal may be obtained by selecting different digital time-out values loaded into a time-out counter. However this type of digital phase shifting or delay device becomes relatively expensive when a large range of phase adjustment and fine resolution are required.
Digital shift registers are also known to be utilized to obtain output signal phase adjustment in phase-locked loops. Here phase adjustment may be obtained by varying the clock frequency of the shift register or the length of the register. However in applications requiring phase adjustment within a wide range and with fine resolution, the implementation is expensive.
The present invention eliminates the above-described disadvantages of known phase control systems by providing an adjustable digital-based phase control system in which the phase of the output signal with respect to a reference signal may be selectively changed with a high degree of resolution, precision, stability and repeatability. At the same time the system of the invention is relatively easy to implement and economical.
In accordance with the present invention a function generator is provided which generates a predetermined periodic waveform having a selected phase displacement with respect to a phase control input signal received by the function generator. A phase controlled oscillator device which provides an output signal having a phase corresponding to the phase relationship between a first and a second input signal thereof, receives as one of its input signals the predetermined waveform from the function generator. The function generator has a phase control input which receives either the output signal from the phase controlled oscillator device or the reference signal. The other input signal of the phase controlled oscillator device is the other one of the output signal or the reference signal, which is not received by the phase control input of the function generator. An adjustable phase displacement control device may be coupled to the function generator for controlling the selected phase displacement of the predetermined waveform.
In the preferred embodiment a digital function generator is utilized in a phase-locked loop. The phase-locked loop has a phase comparator which compares a reference signal to a feedback signal. The error signal from the phase comparator controls a voltage controlled oscillator (VCO). For the purpose of this description we refer to the combination of the phase comparator and VCO as phase-controlled oscillator. The digital function generator generates a sine waveform from stored digital representations of sine waveform samples. The selected phase displacement of the output signal is determined by a phase displacement control device. The function generator generates a waveform from a set of samples representing a sine wave having the selected phase displacement. The VCO provides the output signal from the phase-locked loop which signal in turn is used as a clock for clocking the selected samples in order to produce the sine wave applied from the function generator to the phase comparator as a feedback signal. The resulting feedback signal received by the phase comparator carries the necessary information about the phase of the output signal.
In an alternative embodiment the function generator receives the reference signal at its phase control input and it applies a reference signal having a desired phase displacement to the phase comparator. In this embodiment the phase comparator receives at its other input a feedback signal from the VCO.
It is a significant advantage of the present invention that the phase displacement of the feedback or reference signal is provided by a digital function generator which generates a predetermined periodic signal waveform from stored digital values representing samples of that waveform. A desired phase displacement of that periodic waveform is provided in a digital manner by selecting and storing a set of digital values representing samples which correspond to the phase displaced waveform.
In the preferred embodiment the set of selected digital values representing samples is applied to a digital-to-analog (D/A) converter which in turn generates from these samples a periodic signal having the desired phase displacement. In the preferred embodiment such sets of four digital values representing samples of a sinusoidal waveform are calculated from digital values of a sinusoidal function stored in a read only memory (ROM). The ROM stores 360 such values representing that function in a range from 0 to 90 degrees. Thus a fine resolution of stored values, equal to 1/4 of a degree, is obtained.
The desired phase displacement indicated by the phase displacement control device may be mathematically expressed as: EQU displacement alpha=(k * 90 degrees+gamma) (1)
where k is an integer number in the range of 0 through 3, and gamma is an angle in the range of 0 through 90 degrees. In the preferred embodiment the sine wave is defined by four samples equally spaced from each other by a 90 degree phase displacement. Because of the well known symmetrical properties of the sinusoidal function, the required four samples of the sine wave may be calculated based on well known trigonometrical equations from two values of the sinusoidal function at the angles of gamma degrees and (90-gamma) degrees.
The set of four digital values representing four samples is stored and read from the storage cyclically by a clock signal provided as the VCO output signal.
The thusly read digital valves representing samples are supplied to the D/A converter which produces a step function signal. The step function signal is applied to a bandpass filter which passes the fundamental frequency component of the step function signal as a sinusoidal feedback signal to one input of the phase comparator.
The phase comparator compares the phase of the feedback signal to the previously described reference signal and the resulting error signal therefrom is utilized to control the VCO in order to change the phase of its output signal. This leads to the cancellation of the phase difference between the reference and feedback signal at the input of the phase comparator. In this manner, the output signal from the VCO is maintained at a phase offset from the reference signal by an amount determined by the phase displacement control device.
It is seen from the foregoing description that in accordance with the present invention a desired phase displacement of a feedback or reference signal in a phase-locked loop is obtained by generating a periodic signal waveform having the desired phase displacement using digital-to-analog conversion of stored digital values representing samples of the waveform. The stored values are calculated from digital values of a periodic function corresponding to the desired phase displacement indicated by the phase displacement control device. Because the function generator utilizes substantially digital signal processing techniques, a high degree of stability, accuracy and repeatability while also a desired high degree of resolution of the phase displacement are obtained, as it will also follow from the further detailed description.
In addition to other distinguishing features, the phase control system of the invention thus distinguishes from the previously known systems in that it generates a new periodic signal of a selected waveform, such as sine wave, saw tooth, or other, having a desired phase displacement, for phase comparison in a phase-locked loop. In contrast thereto the known phase control systems utilize phase shifters which merely delay the feedback signal but do not generate a new phase-adjustable feedback signal for the phase comparison.